Ion-selective electrodes (“ISEs”) are sensors that measure the concentration of target ions (or “analytes”) in a gas or solution. For example, the solution is exposed to an ion-selective membrane and a reference electrode. The ion-selective membrane produces an electromotive force (“EMF”) (i.e., a voltage or electrical potential) proportional to the logarithm of the activity (which is approximately equal to the concentration) of the target ions. An ion-to-electron transducer layer may then convert the ion-based potential to an electron-based potential, which in combination with a reference electrode, may be detected by a typical voltage sensor. FIG. 1 illustrates a typical ISE 100 for measuring a concentration [I+]aq of ions in a solution 102. The ISE 100 includes a working electrode 104, a reference electrode 106, an ion-selective membrane 108, and an inner filling solution 110. The electrodes 104, 106 may include or consist essentially of a conductive material (such as platinum, copper, or silver). The working electrode 104 may be coated with an ion-to-electron transducing material, such as silver chloride. The inner filling solution 110 may include or consist essentially of conductive polymers, electrolytic solutions, and/or hydrogels, and may work with the ion-to-electron transducing material to convert ions, passed from the solution 102 through the membrane 108, to electrons. An insulator 112 prevents the solution 102 from coming into direct contact with, for example, the working electrode 104. Assuming that the ion-selective membrane 108 has a known, constant ion concentration [I+]org, the potential measured by the voltage meter 114 is proportional to log([I+]aq/[I+]org), and the unknown concentration [I+]aq may thus be derived from the potential.
One exemplary type of ISE is a single-ion ISE probe, available from a variety of manufacturers, such as Oakton Instruments of Vernon Hills, Ill., and Mettler-Toledo, Inc. of Columbus, Ohio. Single-ion ISE probes typically employ an inner filling solution 110 and a polyvinyl chloride (“PVC”) membrane as the ion-selective membrane 108. These probes are generally cylindrical in shape and relatively large (e.g., approximately 3-4 inches in length and 0.5 inches in diameter) due to issues with manufacturing the inner filling solution.
Commercially available single-ion ISE probes typically have at least two different drawbacks that prevent their use in many applications. First, their relatively large size prohibits their use in size-constrained applications. Single-ion ISE probes are especially unsuitable for applications requiring the simultaneous measurement of multiple analytes in a constrained space because a large number of individual probes is typically required. Second, single-ion ISE probes may be prohibitively expensive, typically exceeding a cost of $400 per probe for a single analyte. In most applications, this high cost eliminates the possibility of a disposable device.
Another exemplary type of ISE, known as a solid-state ISE, features direct contact between the ion-selective membrane 108 and the working electrode 104 (i.e., there is no inner filling solution 110). Alternatively, the inner filling solution 110 may be replaced by some form of a solid material (e.g., a conductive polymer or lipophilic self-assembled monolayer). These all-solid formats allow arrays of solid-state ISEs to be screen-printed or electro-polymerized onto ceramic or plastic substrates.
Existing solid-state ISEs, however, are often unreliable and expensive. One problem occurs at a junction between the ion-selective membrane 108 and the insulating substrate 112. Over time, the aqueous sample solution 102 diffuses through the junction, causing a short circuit. This type of failure is especially common in plasticized PVC ISE membranes printed onto ceramic substrates. Poor material adhesion in such two-dimensional structures renders reliable sealing of the solution 102 difficult.
In addition, while recent research has produced a host of new, solid materials for use in place of the inner filling solution 110, they typically have not proven to be as electrically stable as traditional inner filling solutions described above. FIG. 2 illustrates the typical electrical instability of solid-state ISEs by depicting the response of a micro-electromechanical-system (“MEMS”) based solid-state ISE over time. The initial response curve 202 drifts over time to curve 204 at day five, curve 206 at day fourteen, and curve 208 at day twenty-six. This changing of the response curve over time renders measurements of membrane potential irreproducible and inaccurate.
Thus, in order to service the demand for increasingly functional and accurate (yet disposable) sensor arrays, improved systems and methods for constructing high-density sensors are needed.